1. Field of the Invention
The present invention relates to an exposure control system for a photographic camera, in general, and to a method and apparatus for manually adjusting the nominal exposure time employed in one film exposure cycle for use in a subsequent or second exposure cycle, in particular.
2. Description of the Prior Art
Fully automatic exposure control systems, such as the exposure control system employed in a camera manufactured by Polaroid Corporation of Cambridge, Mass., and sold under its registered trademark "Spectra Camera System" are well known in the art. These automatic exposure control systems have been incorporated within various amateur cameras for simplifying their operations by minimizing the pre-exposure adjustments to be made by a camera operator. These exposure control systems function to automatically regulate the exposure parameters for a given scene. Generally the exposure control systems are designed for use over a wide range of scene lighting conditions.
Since a large majority of amateur pictures are taken within the range of light levels to which the control system is suited, properly exposed photographs are normally obtained. However, the camera operator may wish to take a photograph which is lightened or darkened to his personal taste, may wish to accommodate for unusual lighting effects such as backlighting, highlights or the like, or may wish to account for slight variations in the sensitometric characteristics of the film. In order to adjust the automatic exposure for such personal taste, unusual effects or sensitometric characteristics, a lighten-darken trim system is provided. These trim systems usually supply the camera user with a manual one-half to one stop optional exposure adjustment above or below the exposure value otherwise established automatically.
Two types of trim control systems currently employed in photographic cameras of the self-developing type are described in U.S. Pat. No. 4,209,243, issued June 24, 1982, to Johnson et al, and in U.S. Pat. No. 4,717,935 issued Jan. 5, 1988 to Anacreon. In Johnson et al, the amount of scene light admitted to the film plane of a camera is dependent upon the amount of light passing from the scene to a photocell through a variable density filter slidably mounted on the camera body that varies the amount of scene light sensed by the photocell. The exposure control system may be trimmed to either an underexposure mode upon movement of the slide to reduce the filter density and thus increase the ambient light sensed by the photocell to a value above that indicated for normal exposure, or to an overexposure mode by moving the slide in the opposite direction to increase filter density and thus reduce the amount of light sensed by the cell to a value below that which would pass the filter for normal exposure. In the Anacreon reference, supra, the amount of scene light admitted to the film plane of a camera is also dependent upon the magnitude of an electrical signal generated by an ambient scene light sensing photocell. However, in the trim control apparatus disclosed in Anacreon, the trim adjustment is made by manually increasing or decreasing the magnitude of the electrical signal actually generated by the light sensing photocell representative of ambient scene light, in accordance with the degree of exposure desired above or below a normal exposure.
If after taking a picture of a particular scene, the camera operator determines that the resulting self-developing photograph was either too light or too dark and chooses to take another picture of the particular scene, the particular scene is re-framed and the trim control is manually adjusted. Upon re-framing the scene, the camera's brightness level sensing photocell normally sees a different portion of the same scene and therefore a different ambient scene light level. In such instances, it is the intent of the camera operator to alter the exposure time relating to the initially framed scene that produced the unsuitable scene lighting condition and the improperly exposed photograph and not the exposure time relating to the re-framed scene.